ls: single-column vs. multi-column layout, non-Unicode characters in filenames

Create a directory ~/test with abcdefghijklmnopqrstuvwxyz and zyxwvutsrqponmlkjihgfedcba files in it. ls ~/test will list them using multi-column layout:

abcdefghijklmnopqrstuvwxyz	zyxwvutsrqponmlkjihgfedcba

Each filename is 26 characters long. But if you use Cyrillic letters, the maximum length of a filename for ls to use multi-column layout is limited to 17 characters. If any of your filenames is longer, the single-column layout will be used. Is it documented somewhere? Or is it just a bug?

abcdefghijklmnopqrstuvwxyz		абвгдеёжзийклмноп

abcdefghijklmnopqrstuvwxyz
абвгдеёжзийклмнопр